The present invention relates to wireless communications, and more particularly, to a method of relaying data by a relay station in a time division duplex (TDD)-based wireless communication system.
A transmission scheme used in a wireless communication system can be briefly classified into a frequency division duplex (FDD) scheme and a time division duplex (TDD) scheme. In the FDD scheme, uplink transmission and downlink transmission are achieved in the same time period while using different frequency bands. In the TDD scheme, uplink transmission and downlink transmission are achieved at different time periods while using the same frequency band.
Although the TDD scheme can use all available frequency bands, downlink transmission performed by a base station (BS) and uplink transmission performed by a mobile station (MS) cannot be simultaneously achieved since an uplink transmission time and a downlink transmission time are separated from each other. In a TDD system in which uplink transmission and downlink transmission are divided on a subframe basis, uplink transmission and downlink transmission are performed in different subframes.
A wireless communication system employing a relay station (RS) has recently been developed. The RS is a device that relays data between the BS and the MS, and is used for cell coverage extension and transmission capability improvement.
It is difficult for the RS to receive data from the BS while simultaneously transmitting data to the MS. In addition, it is difficult for the RS to receive data from the MS while simultaneously transmitting data to the BS. This is because a transmit (Tx) signal transmitted by the RS has much greater power than a receive (Rx) signal received by the RS, and thus the Tx signal of the RS may act as interference to the Rx signal, which may result in signal distortion. This is called self interference. In order for the RS to solve the self interference problem, a complex interference cancellation process and spatial separation of Tx/Rx signal processors are required. In reality, it is very difficult for the RS to cancel the self interference, and even if implemented, it will require large expenses. Therefore, it is generally assumed that it is difficult for the RS to simultaneously transmit and receive data by using the same frequency band.
Under the aforementioned assumption, the RS cannot simultaneously perform data reception from the BS and data transmission to the MS (or data transmission to the BS and data reception from the MS), and thus there is a problem in that the TDD system may have low efficiency of resource allocation.
In addition, uplink acknowledgment (ACK) collision is another problem. When the RS transmits data to the BS in a certain uplink subframe, the RS cannot receive data from the MS in the same uplink subframe. Therefore, when the MS transmits an ACK signal to the RS in the uplink subframe, the RS cannot receive the ACK signal. This is called uplink ACK collision. When the uplink ACK collision occurs, the RS cannot know whether the MS successfully receives data transmitted in a downlink subframe prior to the uplink subframe, and thus previously transmitted data may be retransmitted unnecessarily.
The aforementioned problems also occur in a wireless communication system employing a multicast/broadcast single frequency network (MBSFN) subframe-based scheme considered in a 3rd generation partnership project (3GPP) evolved-universal terrestrial radio access network (E-UTRAN). The MBSFN subframe-based scheme is defined as a scheme in which the RS configures a corresponding downlink subframe as an MBSFN subframe when the BS transmits data to the RS.
Accordingly, there is a need for a method capable of effectively relaying data by an RS in a TDD-based wireless communication system.